214 XVI. THE STEM APEX. 



rounded angles. This rhombic figure appears elongated alter- 

 nately in the direction of the newly-entering leaf-traces. The 

 procambium consists of thin-walled, narrow, radially-arranged 

 cells. At the angles of the figure the formation of the elements 

 of the vascular bundles commences ; protophloem elements on 

 the outer, protoxylem on the inner, side of the procambial zone. 

 These regions of commencing differentiation of the elements of 

 the vascular bundles are not delimited towards the rest of the 

 procambium tissue. The procambium zone opens at the places 

 where the foliar bundles enter, in order to admit them. In the 

 axil of each of the young leaves we can see the rudiments of an 

 axillary bud. 



A median longitudinal section, with a very low power, shows 

 the structure of Fig. 83 C. The flat-growing apex, the leaf- 

 rudiments, successively increasing in size, the axillary buds (g), 

 the differentiation of the pith (m), the procambium zone (pc), 

 the vascular bundles, common to leaf and stem (the so-called 

 leaf-traces) (pf) t and the primary cortex (c), are to be seen at a 

 glance. Pith and cortex contain large quantities of cluster- 

 crystals of oxalate of lime. In fresh sections examined in water 

 the pith and cortex appear greenish, while the procambium zone 

 appears bright. In order to follow the arrangement of the cells 

 at the growing apex, we again use clearing reagents. Outer- 

 most on the growing apex we find the unilamellar dermatogen 

 (Fig. 83, D, d) ; under that three sheath of layers, which we have 

 to designate as periblem (pr) ; and then the massive central 

 cylinder of tissue, the plerome (pi), which is not everywhere 

 sharply defined towards the periblem. The growing apex ap- 

 pears very contracted between the two leaf-protuberances last 

 originated ; this is its usual appearance. On the other hand 

 you have often to cut for a long time and many buds before you 

 obtain a section which passes through the first leaf-rudiments. 

 If you are successful in this, the form presented is that of Fig. 

 83 D. The growing apex appears then much broader, and the 

 histogens (or histogenic layers) can be better traced in it. 

 The formation -of the leaves is initiated by periclinal divisions 

 in the two outermost layers of periblem (at/); the dermatogen 

 remains unilamellar. Exactly the same kind of divisions as for 

 the leaf-rudiments take place in the axils of the third youngest 

 pair of leaves, for the formation of axillary buds ; they likewise 

 are initiated by periclinal divisions in the hypodermal layers of 



